1. Field of the Invention
The present invention relates to a low-pressure CVD apparatus (hereinafter referred to as "LP-CVD apparatus") and, more particularly, to a LP-CVD apparatus capable of forming a tungsten blanket layer in improve step coverage.
2. Description of the Prior Art
A nonselective tungsten blanket CVD process (hereinafter referred to as "W-CVD process"), which is capable of filling up minute contact holes and through holes of 0.4 .mu.m square or below in a contact resistance smaller than that of polyplugs formed by filling up minute holes with polycrystalline silicon, has become an attractive technique in the field of the future-generation VLSI industry.
As shown schematically in FIG. 7, a conventional LP-CVD apparatus for forming a tungsten blanket layer has a CVD reaction chamber 6 provided with a source gas inlet 5 through which a source gas 4 is supplied into the CVD reaction chamber 6, a susceptor 1, i.e., a support table, for supporting a wafer or workpiece 2, disposed within the CVD reaction chamber 6, and a shower head 3, i.e., a source gas distributing means, for distributing the source gas 4 over the surface of the wafer 2.
The shower head 3 of this known LP-CVD apparatus of a single-wafer processing system is a ceramic plate having a thickness on the order of about 1 to 2 mm provided with holes of about 2.0 mm in diameter in a random arrangement. The shower head 3 is disposed so as to distribute the source gas 4 uniformly over the wafer 2.
Since the LP-CVD apparatus shown in FIG. 7 does not control the direction of travel of molecules of the source gas 4, molecules of the source gas fall in a substantially isotropic mode into a contact hole (or a trench) 13 as shown in FIG. 8. During this W-CVD process, a seam 14 having a void is formed in the contact hole 13 as shown in FIG. 8, because the tungsten blanket layer 12 grows at substantially the same deposition rate in a vertical direction and in a horizontal direction over a barrier metal layer 11 so that A and B are approximately equal to each other and the aspect ratio of the contact hole 13 increases with the progress of deposition. Thus, it has been impossible to fill up the contact hole 13 perfectly without forming any void therein by the W-CVD process. Accordingly, in etching back the surface of the tungsten blanket layer to form a tungsten plug, the upper portion of the seam is enlarged as shown in FIG. 9 and the enlarged seam forms a void 14a under a wiring layer, not shown, formed over an interlayer insulating film 9, adversely affecting the reliability of the wiring layer.